Datasheet ADP1864 (Analog Devices) - 10

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ADP1864 Data Sheet APPLICATIONS INFORMATION ADIsimPower DESIGN TOOL
SF × PCSV R = (4) The ADP1864 is supported by ADIsimPower design tool set. SENSE(MIN ) I ∆ (PEAK) I + ADIsimPower is a collection of tools that produce complete LOAD(MAX ) 2 power designs optimized for a specific design goal. The tools where SF is the slope factor correction ratio, taken from enable the user to generate a ful schematic, bil of materials, Figure 13, at the system maximum duty cycle (minimum and calculate performance in minutes. ADIsimPower can input voltage). optimize designs for cost, area, efficiency, and parts count
1.05
while taking into consideration the operating conditions and limitations of the IC and all real external components. For
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more information about ADIsimPower design tools, refer to www.analog.com/ADIsimPower. The tool set is available from
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this website, and users can also request an unpopulated board
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through the tool.
TOR C DUTY CYCLE FA 0.65 E
To determine the worst-case inductor ripple current, output
LOP S 0.55
voltage ripple, and slope compensation factor, establish the system maximum and minimum duty cycle. The duty cycle is
0.45
014 calculated by the equation 05562-
0.35
+
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Duty Cycle (DC) OU V T D V = (1)
DUTY CYCLE
+ IN V D V Figure 13. Slope Factor (SF) vs. Duty Cycle where VD is the diode forward drop.
INDUCTOR VALUE
A typical Schottky diode has a forward voltage drop of 0.5 V. The inductor value choice is important because it dictates the
RIPPLE CURRENT
inductor ripple and, therefore, the voltage ripple at the output. Choose the peak-to-peak inductor ripple current between 20% When operating the part at >40% duty cycle, keep the inductor and 40% of the maximum load current at the system’s highest value low enough for the slope compensation to remain input voltage. A good starting point for a design is to pick the effective. peak-to-peak ripple current at 30% of the load current. The inductor ripple current is inversely related to the ΔI inductor value. (PEAK) = 0.3 × ILOAD(MAX) (2) (V − IN V )
SENSE RESISTOR
 V + V  ∆I = OUT ×  OUT D  (PEAK )   (5) Choose the sense resistor value to provide the desired current L × f  V + IN VD  limit. The internal current comparator measures the peak where f is the oscillator frequency. current (sum of load current and positive inductor ripple current) and compares it against the current limit threshold. Smaller inductor values are usually less expensive, but increase The current sense resistor value is calculated by the equation the ripple current and the output voltage ripple. Too large an inductor value results in added expenses and can impede effective PCSV = (3) load transient responses at >40% duty cycle because it reduces S R ENSE(MIN) I ∆ (PEAK) I + the effect of slope compensation. LOAD(MAX ) 2 Start with the highest input voltage, and assuming the ripple where PCSV is the peak current sense voltage, typically 0.125 V. current is 30% of the maximum load current, To ensure the design provides the required output load current (V − IN V )  V + V  over al system conditions, consider the variation in PCSV over L = OUT ×  OUT D    (6) temperature (see the Specifications section) as well as increases 0.3 × ILOAD(MAX) × f  V + IN D V  in ripple current due to inductor tolerance. From this starting point, modify the inductance to obtain If the system is being operated with >40% duty cycle, incor- the right balance of size, cost, and output voltage ripple, while porate the slope compensation factor into the calculation. maintaining the inductor ripple current between 20% and 40% of the maximum load current. Rev. C | Page 10 of 16 Document Outline Features Applications General Description Typical Applications Diagram Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Loop Startup Short-Circuit Protection Undervoltage Lockout (UVLO) Overvoltage Lockout Protection (OVP) Soft Start Applications Information ADIsimPower Design Tool Duty Cycle Ripple Current Sense Resistor Inductor Value MOSFET Diode Input Capacitor Output Capacitor Feedback Resistors Layout Considerations Example Applications Circuits Outline Dimensions Ordering Guide